1. Field of the Invention
The present invention relates to a dielectric device and a method of manufacturing a dielectric device, and more particularly, it relates to a dielectric device having a dielectric film formed on an electrode layer and a method of manufacturing such a dielectric device.
2. Description of the Background Art
In general, a dielectric device having a ferroelectric film is expected for application to various fields such as that of electronics due to its characteristics such as ferroelectricity. For example, research is made on a nonvolatile ferroelectric memory utilizing polarization hysteresis or the like.
A conventional ferroelectric film is manufactured through a dry process or a wet process. The dry process includes sputtering, vacuum deposition, laser absorption, MOCVD (metal organic chemical vapor deposition) or the like. In the dry process, however, it is difficult to stably form a ferroelectric film close to stoichiometry due to vapor pressures varying with elements. Further, the ferroelectric film is disadvantageously deteriorated in crystallinity. In addition, the manufacturing cost is disadvantageously increased due to low productivity and high-priced equipment. At present, therefore, the dry process is not yet put into practice.
According to the wet process, on the other hand, the chemical composition of the ferroelectric film can be precisely controlled and the ferroelectric film can be homogeneously formed at the molecular level as compared with the dry process. Further, it is also possible to form a ferroelectric film having a large area. In addition, the manufacturing cost can be reduced due to low-priced equipment, and the wet process is suitable for mass production. In general, therefore, the wet process is employed for forming a ferroelectric film on a relatively flat substrate. This wet process includes a sol-gel process employing a metal organic compound or MOD (metal organic decomposition) employing a solution prepared by dissolving metallic salt of carboxylic acid or the like in an organic solvent. The sol-gel process and the MOD are now schematically described.
According to the sol-gel process, a source solution containing a hydrolytic compound of each constituent metal serving as a raw material, a partial hydrolysate of the hydrolytic compound or a partial polycondensate of the hydrolytic compound is applied to a substrate. The coating of the source solution formed on the substrate is dried and thereafter heated at a prescribed temperature in the air or a water vapor atmosphere, thereby forming a film consisting of a metallic oxide on the substrate. Then, the film consisting of the metallic oxide formed on the substrate is baked at a temperature exceeding the crystallization temperature for the metallic oxide, thereby crystallizing the film of the metallic oxide. If the metallic oxide is SBT, the film is crystallized by performing heat treatment at a temperature of about 750° C.
According to the MOD, on the other hand, a source solution containing a heat-decomposable organic metal compound is applied to a substrate. The coating of the source solution formed on the substrate is heated in an atmosphere containing oxygen, thereby evaporating a solvent contained in the coating and thermally decomposing the metal compound. Thus, a film consisting of a metallic oxide is formed on the substrate. Then, the film consisting of the metallic compound formed on the substrate is baked at a temperature exceeding the crystallization temperature for the metallic oxide, thereby crystallizing the film of the metallic oxide. If the metallic oxide is SBT, the film is crystallized by performing heat treatment at a temperature of about 750° C.
In a conventional dielectric device, an electrode layer is formed on a substrate, and a ferroelectric film is thereafter formed on the electrode layer by the aforementioned sol-gel process or MOD.
FIG. 10 is a model diagram for illustrating the crystal structure of a ferroelectric film (SBT film) 105 formed on a lower electrode 104 consisting of Pt by the conventional sol-gel process or MOD. Referring to FIG. 10, elements shown by large characters are in the same plane, and those shown by small characters are in a plane different from that of the elements shown by the large characters.
When the ferroelectric film 105 consisting of SrBi2Ta2O9 (SBT) having a bismuth layer structure is formed on the lower electrode 104 consisting of Pt by the sol-gel process or the MOD as shown in FIG. 10, the ferroelectric film 105 consisting of SBT generally exhibits a layer structure including a pseudo-perovskite layer 105a held between bismuth oxide layers 105b. 
It is known that the a-axis direction and the b-axis direction of a ferroelectric substance having a bismuth layer structure exhibit polarization capacities larger than that in the c-axis direction. In order to obtain a dielectric device having excellent ferroelectricity, therefore, it is preferable to form a ferroelectric film on an electrode layer so that the a-axis direction and the b-axis direction are perpendicular to the surface of the electrode layer.
If the ferroelectric film 105 consisting of SBT having a bismuth layer structure is formed on the lower electrode 104 consisting of Pt by the conventional sol-gel process or MOD, however, the ferroelectric film 105 is so easily formed on the lower electrode 104 that the a-axis direction and the b-axis direction of the ferroelectric film 105 are parallel to the surface of the lower electrode 104 as shown in FIG. 10. This is conceivably for the following reason: Bi constituting the bismuth oxide layers 105b easily bonds to Pt constituting the lower electrode 104 due to catalytic action of Pt. When Bi bonds to Pt in this manner, the bismuth oxide layers 105b are conceivably easily formed to extend in parallel with the surface of the lower electrode 104 consisting of Pt. In this case, the a-axis direction and the b-axis direction of the ferroelectric film 105 are parallelized to the surface of the lower electrode 104.
As described above, the a-axis direction and the b-axis direction of the ferroelectric film 105 are parallelized to the surface of the lower electrode 104 consisting of Pt when a wet process such as the sol-gel process or MOD is employed, and hence it is difficult to attain excellent ferroelectricity in general.
In the conventional method of forming a ferroelectric film on an electrode layer by the sol-gel process or MOD, further, heat treatment must disadvantageously be performed at a high temperature of about 750° C. for crystallizing a metal oxide (ferroelectric substance), as hereinabove described.